An entirely new theory of aortic root function, recently developed at our institution, suggests that the normal aortic root is a dynamic structure integrated with the leaflets and functioning as a single unit. It is probable that during the opening and closing of the aortic valve, the leaflets change very little in length in the circumferential direction, and that their motion is achieved by increasing and decreasing diameter of the aortic root along the attachment of the leaflets. These changes in diameter would occur in response to the normal cyclic variations in blood pressure with cardiac contraction, resulting in a system in which the flexion stresses and tensions within the leaflets are far less than previously postulated. The objective of the present proposal is to determine the detailed structure and function of naturally occuring semilunar valves, specifically the aortic valve and aortic root and venous valves. The aortic root and valve of several species of animals will be studied by (1) Silicone rubber casting at physiologic pressures in the systolic and diastolic positions; (2) Measurement of the dynamic changes in the dimensions of the aortic root and leaflets during the cardiac cycle in a pulse duplicator, and in vivo; and cineradiographic study of the normally functioning aortic roots, after the roots and leaflets are marked with radiopaque material. Venous valves will be studied by similar methods. This information will be used to develop theories of the pathology and origin of congenital anomalies of the venous and aortic valves, and should assist in the further development of devices for valvular replacement. Finally, this information will be used to determine whether venous valves are deleterious when veins are used as arterial substitutes.